FIG. 1 shows a cross-sectional view of a cold-cathode fluorescent lamp in a backlight module according to the prior art. The fluorescent lamp 100 comprises a glass tube 120, which includes a pair of cup-shaped metallic electrodes 110 inserted in its both ends and two leads 130 connected to the ends of the two metallic electrodes 110. While manufacturing the fluorescent lamp 100, even the fluorescent lamp 100 is pumped to a certain vacuum level, primary electrons still naturally appear therein owing to the appearance of cosmic rays. In the fabrication process of the fluorescent lamp 100, after vacuuming, the fluorescent lamp 100 is filled with a neon-argon (Ne—Ar) gas 150 in a pressure above 50 torr. When a high AC voltage is applied to the metallic electrodes 110 at both ends of the fluorescent lamp 100, the primary electrons will be accelerated by electric field and hence ionizing the Ne—Ar gas 150. When the ionization persists, spark plasma is formed, in which cations 160 and negative electrons 140 coexist. The cations 160 and the electrons 140 scatter the two metallic electrodes 110 and thereby are neutralized. Under such circumstance, secondary electrons are produced from the two metallic electrodes 110 owing to the scattering and thus enabling continuous discharging. Accordingly, production of secondary electrons is a significant factor for implementing continuous light emission. If emission of secondary electrons is supported, high brightness will be maintained.
When the electrons 140 scatter neutral mercury atoms 170, the latter will be excited. When the excited mercury atoms 170 return to the ground state, they can emit UV light 180. The UV light 180 will emit to the phosphorus 190 coated on the inner sidewalls of the glass tube 120, and thus converted to visible light 181. Thereby, the electrons 140 or the cations 160 bombard the metallic electrodes 110 and sputter there. The scattered metallic electrode material after sputtering will adhere to the mercury atoms 170 and form a complex. When the complex is deposited around the metallic electrodes 110, darkening phenomenon occurs, which leads to shortening of the lifetime of the fluorescent lamp 100 and bring a major issue to the fluorescent lamp 100.
To overcome the problem, several methods are proposed. (1) A method for reducing the initial discharge voltage by using the Penning effect according to the stimulation ad ionization of the Ne—Ar gas 150 filled in the fluorescent lamp 100. Thereby, bombardments of the electrons 140 or cations 160 on the metallic electrodes 110 can be reduced, and thus weakening sputtering. (2) A method for reducing the initial discharge voltage by lowering the air pressure to the possible lowest. Nonetheless, when the initial discharge voltage is very low, the kinetic energy on the cations 160 or the electrons 140 bombarding the metallic electrodes 110 is reduced and thus reducing emission of secondary electrons from the metallic electrodes 110. Consequently, the brightness of the fluorescent lamp 100 is weakened.
For conquering this problem, another method is proposed. This method selectively adopts materials with low a work function as the metallic electrodes 110 for facilitating electron supply. Nevertheless, this method will increase the manufacturing cost owing to the costly price of the materials. In addition, this method also need to use expensive borosilicate as the material of the glass tube 120 for adjusting the heat expansion coefficients of the glass tube 120 and the leads 130. Moreover, the fluorescent lamp 100 has a low resistivity, and thereby its resistive component will be obviously high. Hence, one transformer can only drive a single fluorescent lamp 100, resulting in overall manufacturing cost increased. Besides, because the diameter of the glass tube 120 is increased, brightness will be drastically reduced and the mechanical strength of the fluorescent lamp 100 is relatively weaker. Accordingly, the fluorescent lamp 100 described above is not easy to be applied to large-size televisions that need a large-diameter fluorescent lamp (with a diameter greater than 4 mm) as the backlight.
To solve the problem describe above, a fluorescent lamp having external electrodes is developed. As shown in FIG. 2, conductive layers 221 are disposed on the outer surface of both ends of the glass tube 210, respectively. Alternatively, both ends of the glass tube 210 are covered by and contact with metallic caps 220, respectively. According to the fluorescent lamp 200 having external electrodes shown in FIG. 2, phosphorus is coated on the inner surface of the glass tube 210 and both ends thereof are sealed. The inner space of the glass tube 210 is filled with mixture containing charged gas, including, for example, inert gas such as Ar or Ne and mercury (Hg) gas. The conductive layers 221 have various shapes and are disposed on the outer surface of both ends of the glass tube. They can be made of silver or carbon. Beside, metallic caps 220 are disposed on both ends of the glass tube 210, respectively.
When a high AC voltage is applied to the conductive layers 2210, both ends of the glass tube 210 contacting with the metallic caps 220 act as a dielectric material for producing a strong induced electric field. More specifically, when the polarity of the voltage applied to the metallic cap 220 is positive, electrons are accumulated in the glass tube 210 contacting with the conductive layer 221. On the other hand, when the voltage is negative, cations are accumulated in the glass tube 210 contacting with the conductive layer 221. Because AC electric field changes polarities continuously, the charges accumulated on the sidewalls of both ends of the glass tube 210 interchange. Hence, when the charges on the sidewalls bombard the Hg gas supplied along with the inert gas, the Hg atoms will be excited. Then, the UV light produced during this excitation process can excite the phosphorus coated on the inner sidewalls of the glass tube 210 and thus emitting visible light.
In a conventional fluorescent lamp 200 with external electrodes, because the regions at both ends of the glass tube 210 act as a dielectric material and have the conductive layer 221, the end regions will be enlarged and hence increasing the amount of sidewall charges and, in turn, increasing the brightness of the fluorescent lamp 200. Nonetheless, the conductive layer 221 is limited while extending in the longitudinal direction. Thereby, the radiated light of the conductive layer 221 will be reduced in the longitudinal direction, leading to reduction of light-emitting efficiency.
Owing to the drawbacks described above, Taiwan patent publication number 200842928 entitled “Fluorescent Lamp Having Ceramic-Glass Composite Electrode” disclosed a ceramic-glass composite electrode, which is a composite of ceramic and glass having a higher dielectric constant and a better secondary electron emission efficiency. In addition, the ceramic-glass composite electrode owns higher polarity under the same electric field, and thereby more electrons and cations can be moved, resulting in improved brightness of the fluorescent lamp. As shown in FIG. 3, the ceramic-glass composite electrode 300 exhibits a hollow cylindrical shape to be disposed at both ends of the glass tube. The ceramic-glass composite electrode 300 has two different inner radii 310, 313, in which the inner radius 310 is smaller than the inner radius 313. Accordingly, the inner side of the ceramic-glass composite electrode 300 is ladder-shaped. The inner radius 313 is slightly larger than the outer radius of the glass tube for allowing the ceramic-glass composite electrode 300 to slip on the end of the glass tube. Besides, the inner radius 310 is smaller than the outer radius of the glass tube.
Before the ceramic-glass composite electrode 300 slips on the glass tube, the outer surface at the end of the glass tube has to be coated with an adhesive and the ceramic-glass composite electrode 300 is disposed. Nonetheless, the dose of coating adhesive on the outer surface of the glass tube is difficult to be controlled. Thereby, excess or insufficient adhesives tend to be applied. If the adhesive is insufficient, the ceramic-glass composite electrode 300 cannot be fixed at the end of the ceramic-glass composite electrode 300 firmly; if excess adhesive is applied, it will spill into the glass tube, and thus contaminating the gas mixture in the glass tube and affecting the light-emitting efficiency and lifetime of the fluorescent lamp. In addition, because the inner radii of the ceramic-glass composite electrode 300 are different, it is difficult to fabricate, which means that process complexity and costs are increased. Thereby, how to prevent adhesives from flowing into the glass tube while slipping the ceramic-glass composite electrode 300 on the end of the glass tube has become a major issue at present.
Accordingly, the present invention provides a ceramic-glass composite electrode and a fluorescent lamp having the same for solving the problems described above. The present invention not only improves the above-mentioned drawbacks appeared in the prior art but also extends the lifetime of the fluorescent lamp.